Drive circuit and light emitting device

ABSTRACT

A drive circuit  3  includes a power source  11 ; current control units  12 - 1  to  12 - n  configured to control the amount of currents supplied to a light emitting element in accordance with a pulse modulation signal; and a calculation unit  13  configured to change a duty ratio of a pulse modulation signal. The current control units  12 - 1  to  12 - n  include a first switching element  21  configured to be switched on/off in accordance with a pulse modulation signal; and a second switching element  22  configured to be switched on/off in accordance with an inversion signal of the pulse modulation signal input to the first switching element  21 ; and an inductor  23 . The first switching element  21  and the inductor  23  are serially connected between the power source and the light emitting element. The second switching element  22  is connected between ground  25  and a contact point  24  of the first switching element  21  and the inductor  23 . The two or more current control units  12 - 1  to  12 - n  are connected in parallel.

TECHNICAL FIELD

The present invention relates to a drive circuit and a light emittingdevice.

BACKGROUND ART

Light emitting devices drive a light emitting element such as a laserdiode (hereafter, abbreviated as LD) or LED (Light Emitting Diode), andthey include a drive circuit that supplies a drive current to the lightemitting element. Drive circuits have a known configuration such that,for example, a constant-voltage source, a light emitting element, aswitching element (e.g., MOSFET (Metal-Oxide-Semiconductor Field-EffectTransistor) or a bipolar transistor) are connected and the value ofcurrent supplied to the light emitting element is controlled. Opticaloutput of the light emitting element is determined in accordance withthe value of current flowing into the PN junction area of asemiconductor inside the light emitting element. Drive currents aresupplied as direct currents or as pulses depending on a use application.

Known methods for controlling the current value of a drive currentinclude a continuous control method (sometimes referred to as an analogcontrol method, a linear method, a drop method, a dropper method, or thelike) for continuously controlling a gate voltage of a switching elementby using an analog control signal; and a switching control method forswitching on/off a gate voltage of a switching element by using a pulsemodulation signal. According to the continuous control method, forexample, a constant-voltage source, a light emitting element, and aswitching element are connected in series, and the gate voltage of theswitching element is controlled in a continuous manner. Thus, theswitching element is used as a pseudo variable resistance so that thecurrent value of a drive current is controlled. According to theswitching control method, for example, an inductor is provided among aconstant-voltage source, a light emitting element, and switchingelements and the switching elements are turned on/off at an appropriateduty ratio by using a pulse modulation signal so that the current valueof the drive current is controlled. Furthermore, diode rectification isknown in which one of the switching elements is replaced with a diode ina switching method. The switching control method is advantageous inelectric-power conversion efficiency, size, and the like, as typicallyit has little circuit loss than the continuous control method.

There is a disclosure of the configuration of a drive circuit using aswitching control method, including a current output unit that controlsa switching element inside a step-down chopper unit that reduces adirect-current voltage such that a detected current value matches adesignated current value, the switching element being connected to thelight emitting unit in parallel (PTL 1).

SUMMARY OF INVENTION Technical Problem

If an inductor is used in a switching control method, there is a need tomake consideration to prevent the occurrence of magnetic saturation inthe inductor. If magnetic saturation occurs in the inductor (forexample, if the magnetic flux density of the core material of a coilreaches a saturation magnetic flux density), inductance rapidlydecreases, and the amount of current flowing from the inductor rapidlyincreases; therefore, the current flowing into the switching elementconnected to the inductor exceeds the rated one, which may result indamages to the switching element. Therefore, it is necessary to use aninductor with a large saturation magnetic flux density to prevent theoccurrence of magnetic saturation so that the current value of theoutput current supplied to a light emitting element becomes larger. Toincrease a saturation magnetic flux density, there is a need to increasea magnetic path length (raise a core volume), and therefore the size ofthe inductor becomes larger.

The present invention has been made in consideration of the foregoing,and it has an object to increase an output current without increasingthe size of an inductor.

Solution to Problem

According to an embodiment, provided is a drive circuit configured togenerate an output current for driving a light emitting element,including: a power source; a current control unit configured to controlan amount of current supplied to the light emitting element inaccordance with a pulse modulation signal; and a calculation unitconfigured to change a duty ratio of the pulse modulation signal,wherein the current control unit includes a first switching elementconfigured to be switched on/off in accordance with the pulse modulationsignal, a second switching element configured to be switched on/off inaccordance with an inversion signal of the pulse modulation signal inputto the first switching element, and an inductor, the first switchingelement and the inductor are serially connected between the power sourceand the light emitting element, the second switching element isconnected between ground and a contact point of the first switchingelement and the inductor, and the two or more current control units areconnected in parallel.

Advantageous Effects of Invention

According to the present invention, it is possible to increase an outputcurrent without increasing the size of an inductor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram that illustrates a configuration of a light emittingdevice according to a first embodiment.

FIG. 2 is a graph that illustrates the relation between the outputcurrent and electric-power conversion efficiency in a voltage conversioncircuit with a switching control method.

FIG. 3 is a graph that illustrates the relation between the outputcurrent and electric-power conversion efficiency when four voltageconversion circuits with the same switching control method are arrangedin parallel.

FIG. 4 is a timing chart that illustrates the relation among the targetcurrent value, the timing signals, the inductor current, and the outputcurrent according to the first embodiment.

FIG. 5 is a graph that illustrates the relation between the duty ratioof the timing signal and the inductor current according to the firstembodiment.

FIG. 6 is a graph that illustrates the relation between the outputcurrent and a forward voltage of an LD according to the firstembodiment.

FIG. 7 is a diagram that illustrates a state where duty ratios,inductance, control-unit internal resistances, a forward voltage, an LDinternal resistance, and a threshold voltage are described in theconfiguration diagram of the light emitting device illustrated in FIG.1.

FIG. 8 is a diagram that illustrates the relation among the targetcurrent value with regard to the light emitting device according to thefirst embodiment, the duty ratio, the inductor current, and the outputcurrent.

FIG. 9 is a diagram that illustrates a state where a first switchingelement in an n-th current control unit according to the firstembodiment is off and a second switching element is on.

FIG. 10 is a diagram that illustrates a state where the first switchingelement and the second switching element in the n-th current controlunit according to the first embodiment are off.

FIG. 11 is a timing chart that illustrates the relation among the targetcurrent value, the duty ratio, the timing signals, the inductor current,and the output current in the state illustrated in FIG. 10.

FIG. 12 is a diagram that illustrates a configuration of the lightemitting device according to a modification of the first embodiment.

FIG. 13 is a graph that illustrates the relation among the duty ratio,the output current, and the number of current control units to be drivenaccording to the first embodiment.

FIG. 14 is a timing chart that illustrates control if the number of fourcurrent control units to be driven is sequentially decreased accordingto the first embodiment.

FIG. 15 is a timing chart that illustrates control if the four currentcontrol units according to the first embodiment are sequentially stoppedone by one.

FIG. 16 is a timing chart that illustrates control if the target currentvalue is periodically changed according to the first embodiment.

FIG. 17 is a diagram that illustrates a configuration of the lightemitting device according to a second embodiment.

FIG. 18 is a timing chart that illustrates operation when a drivecircuit according to a second embodiment performs a failure detectionfunction.

FIG. 19 is a timing chart that illustrates operation when the drivecircuit according to the second embodiment performs a failure handlingfunction.

FIG. 20 is a diagram that illustrates a configuration of the lightemitting device according to a third embodiment.

DESCRIPTION OF EMBODIMENTS

With reference to the attached drawings, a detailed explanation is givenbelow of embodiments of a drive circuit and a light emitting device. Thepresent invention is not limited to the embodiments below, andcomponents in the embodiments below include the ones that may be easilydeveloped by a person skilled in the art, substantially the same ones,and the ones in what is called a range of equivalents. The componentsmay be variously omitted, replaced, modified, or combined withoutdeparting from the scope of the embodiments below.

First Embodiment

FIG. 1 is a diagram that illustrates a configuration of a light emittingdevice 1 according to a first embodiment. The light emitting device 1includes an LD 2 (light emitting element) and a drive circuit 3. The LD2 is a light emitting element that is driven with an output currentI_(o) output from the drive circuit 3.

The drive circuit 3 according to the present embodiment includes adirect-current power source 11 (power source), multiple current controlunits 12-1 to 12-n, a calculation unit 13, and a capacitor 14. The drivecircuit 3 is a circuit that generates the output current I_(o) by usinga switching control method.

The direct-current power source 11 conducts voltage conversion on the ACvoltage supplied from a commercial outlet, or the like, or the DCvoltage supplied from a battery, or the like, in accordance with thevoltage used by the drive circuit 3. The direct-current power source 11generates an input voltage V_(in).

The two or more current control units 12-1 to 12-n are connected inparallel between the direct-current power source 11 and the LD 2. Thecurrent control units 12-1 to 12-n are circuits that control the amountof the output current I_(o) in accordance with pulse modulation signals.Each of the current control units 12-1 to 12-n includes a firstswitching element 21, a second switching element 22, and an inductor 23.The first switching element 21 and the inductor 23 are seriallyconnected between the direct-current power source 11 and the LD 2. Thesecond switching element 22 is connected between a ground 25 and acontact point 24 of the first switching element 21 and the inductor 23.

The first switching element 21 and the second switching element 22according to this example are n-type MOSFET whose on/off state isswitched by timing signals PWMH, PWML that are pulse modulation signalsoutput from the calculation unit 13. The first switching element 21 iscontrolled by the timing signal PWMH, and the second switching element22 is controlled by the timing signal PWML that is an inversion signalof the timing signal PWMH. Here, the timing signal PWMH and the timingsignal PWML do not always have an inversion relation and for example thesignals PWMH, PWML sometimes have an identical potential at the sametime.

The calculation unit 13 is a circuit that outputs the timing signalsPWMH, PWML (pulse modulation signals) for controlling the gate voltagesof the first switching element 21 and the second switching element 22.The calculation unit 13 controls the pulse width (duty ratio) of thetiming signals PWMH, PWML in accordance with the target current value ofthe output current I_(o). The calculation unit 13 may be configured byusing, for example, a voltage control IC (integrated circuit), a currentcontrol IC, a microcomputer, or FPGA (Field-Programmable Gate Array).The microcomputer and the FPGA may be configured by using a CPU (CentralProcessing Unit), a ROM (Read Only Memory) that stores programs forcontrolling the CPU, a RAM (Random Access Memory) that is a work areafor the CPU, or the like.

The inductor 23 has a function to store currents output from the firstswitching element 21 and smooth the output current I_(o). The inductor23 needs to be used in such a range that no magnetic saturation occurs.This is because if magnetic saturation occurs in the inductor 23, i.e.,if the magnetic flux density of the core material reaches a saturationmagnetic flux density, the inductance rapidly decreases, and the amountof inductor currents i[1] to i[n] flowing from the inductor 23 rapidlyincreases so that the current flowing into an element (the firstswitching element 21, the second switching element 22, or the like)connected to the inductor 23 exceeds the rated one, which may result indamages to the elements.

In order to supply a sufficient amount of the output current I_(o) tothe LD 2, the core of the inductor 23 needs to be selected so that themagnetic flux density does not exceed a saturation magnetic flux densitywhile the desired inductance is obtained. The following Equation (1) andEquation (2) are provided where the inductor current is i, theinductance is L, the magnetic flux density is B, the saturation magneticflux density is B_(max), the number of turns of the core is N, themagnetic path length is l_(e), the cross-sectional area of the inductor(coil) 23 is A_(e), and the magnetic permeability is μ.

$\begin{matrix}{L = {N^{2} \cdot \frac{\mu \cdot A_{e}}{l_{e}}}} & (1) \\{\beta = {{N \cdot i \cdot \frac{\mu}{l}} \leq B_{\max}}} & (2)\end{matrix}$

The inductance L is proportional to the square of the number of turns N,and the number of turns N needs to be increased to obtain the desiredinductance L. However, as the magnetic flux density B is defined by theproduct of the number of turns N and the inductor current i, an increasein the number of turns N and an increase in the inductor current i causethe saturation magnetic flux density B_(max) to be exceeded, whichresults in core saturation. Furthermore, as the inductor current iincreases, loss (copper loss) caused due to resistance of a winding wireitself increases, and the temperature of the inductor 23 rises. Anincrease in the temperature of the inductor 23 causes a decrease in thesaturation magnetic flux density B_(max). Therefore, to prevent magneticsaturation while the desired inductance L is obtained, there is a needto increase the magnetic path length l_(e), i.e., raise the core volume.There is, however, a problem in that, for the high output current I_(o),the volume of the inductor 23 is excessively large. Therefore, accordingto the present embodiment, as the current control units 12-1, 12-2, . .. , 12-n including the inductors 23 are arranged in parallel, the highoutput current I_(o) is achieved while an increase in the size of theindividual inductor 23 is prevented.

The output current I_(o) is the synthesis of the inductor currents i[1]to i[n] output from the respective current control units 12-1 to 12-n.That is, the output current I_(o) is represented by the followingEquation (3).

Io=i _([1]) +i _([2]) +Λ+i _([n])=Σ_(k=1) ^(n) i _([k])  (3)

The capacitor 14 is connected to the LD 2 in parallel, and it has thefunction to control ripples of the output current I_(o). Although ripplecurrents need to be controlled so as not to exceed the maximum allowablecurrent magnitude of the LD 2, it is sometimes not necessary to controlit in some use situations. Therefore, if control on ripple currents isnot necessary, the capacitor 14 does not need to be provided.

FIG. 2 is a graph that illustrates the relation between the outputcurrent I_(o) and electric-power conversion efficiency η in a voltageconversion circuit with a switching control method. FIG. 2 illustratesthat the electric-power conversion efficiency η has the single maximumvalue (maximum electric-power conversion efficiency η_(max)) at acurrent value Iη_(max).

FIG. 3 is a graph that illustrates the relation between the outputcurrent I_(o) and the electric-power conversion efficiency η when fourvoltage conversion circuits with the same switching control method arearranged in parallel. FIG. 3 illustrates that the electric-powerconversion efficiency η has four maximum values when the four voltageconversion circuits are driven. In this way, multiple voltage conversioncircuits are arranged in parallel, and the number of voltage conversioncircuits in operation is changed in accordance with the target currentvalue of the output current I_(o) so that the high electric-powerconversion efficiency η can be retained in a wide range of the currentvalue of the output current I_(o).

Therefore, in the drive circuit 3 according to the present embodiment,the current control units 12-1 to 12-n are connected in parallel with aswitching control method so that the current value of each of thecurrent control units 12-1 to 12-n is decreased and the high outputcurrent I_(o) is achieved without increasing the size of the inductor23. Thus, the output current I_(o) of a large current value (e.g., a fewhundred A) can be output without causing magnetic saturation in theinductor 23. Furthermore, the drive states of the current control units12-1 to 12-n are controlled in accordance with the target current valueof the output current I_(o) so that high outputs can be achieved whilethe high electric-power conversion efficiency η is retained.

FIG. 4 is a timing chart that illustrates the relation among the targetcurrent value Ictrl, the timing signals PWMH, PWML, the inductor currenti, and the output current I_(o) according to the first embodiment. Thetarget current value Ictrl input from an external device to thecalculation unit 13 is, for example, an analog signal whose currentvalue is determined in accordance with a voltage or a digital signalthat uses I2C (registered trademark), or the like. After the targetcurrent value Ictrl is input to the calculation unit 13, the duty ratiosof the timing signals PWMH[1] to PWMH[n], PWML[1] to PWML[n] fed to therespective current control units 12-1 to 12-n are adjusted in accordancewith the target current value Ictrl.

The lower section in FIG. 4 illustrates an enlarged view of the timingsignals PWMH[n], PWML[n]. The duty ratio D[n] of the timing signalPWMH[n] is D[n]=Ton[n]/T, where the ON time of the timing signal PWMH[n]to the first switching element 21 is Ton[n] and a cycle is T.Furthermore, as the timing signal PWML[n] to the second switchingelement 22 is an inversion signal of the timing signal PWMH[n] to thefirst switching element 21, the duty ratio of the timing signal PWML[n]is 1−D[n].

FIG. 5 is a graph that illustrates the relation between the duty ratio Dof the timing signal PWMH and the inductor current i according to thefirst embodiment. The inductor currents i, i.e., currents output fromthe current control units 12-1 to 12-n, are linearly changed inaccordance with the duty ratio D of the timing signal PWMH to the firstswitching element 21. Furthermore, according to this example, when theinductor current i is 0, the duty ratio D≠0; however, this relation isan example, and changes are made in accordance with the characteristicsof the LD 2.

FIG. 6 is a graph that illustrates the relation between the outputcurrent I_(o) and a forward voltage V_(f) of the LD 2 according to thefirst embodiment. The forward voltage V_(f) is a voltage at both ends ofthe LD 2 in a forward direction with respect to the output currentI_(o). The forward voltage V_(f) of the LD 2 changes in accordance withthe output current I_(o). After the output current I_(o) exceeds acertain value, the forward voltage V_(f) linearly changes with regard tothe output current I_(o). Typically, the drive current (the outputcurrent I_(o)) of the LD 2 is used in such a linear area. Therefore, thecurrent differential value of the forward voltage V_(f) in a linear areais γ_(d)=ΔV_(f)/ΔI_(o), and the forward voltage V_(f) is a thresholdvoltage V_(f0) when the output current I_(o)=0, obtained from anapproximation line in the linear area. The threshold voltage V_(f0)occurs due to a potential barrier of the LD 2. Here, the forward voltageV_(f) is represented by the following Equation (4) where an LD internalresistance, which is an internal resistance of the LD 2, is r_(d).

V _(f) =r _(d) ·Io+V _(f0)  (4)

FIG. 7 is a diagram that illustrates a state where duty ratios D[1] toD[n], 1-D[1] to 1-D[n], inductance L[1] to L[n], control-unit internalresistances r_(L)[1] to r_(L)[n], a forward voltage V_(f), an LDinternal resistance r_(d), and a threshold voltage V_(f0) are describedin the configuration diagram of the light emitting device 1 illustratedin FIG. 1.

The duty ratios D[1] to D[n] are calculated by the calculation unit 13,and each of them operates in the corresponding first switching element21. The duty ratios 1-D[1] to 1-D[n] are calculated by the calculationunit 13, and each of them operates in the corresponding second switchingelement 22. The inductance L[1] to L[n] represents the inductance of theinductor 23 included in each of the current control units 12-1 to 12-n.The control-unit internal resistances r_(L)[1] to r_(L)[n] represent therespective internal resistances of the current control units 12-1 to12-n. The control-unit internal resistances r_(L)[1] to r_(L)[n]correspond to parasitic resistances due to the inductance L[1] to L[n]in the current control units 12-1 to 12-n, wiring resistance, and thelike. The forward voltage V_(f) represents a voltage at both ends of theLD 2 in a forward direction with respect to the output current I_(o).The LD internal resistance r_(d) represents the internal resistance ofthe LD 2. The threshold voltage V_(f0) represents a voltage due to apotential barrier of the LD 2.

The output current I_(o) is calculable by the following Equation (5)according to a state averaging technique by using the input voltageV_(in) of the direct-current power source 11, the number n of thecurrent control units 12-1 to 12-n, the control-unit internalresistances r_(L)[1] to r_(L)[n], the duty ratios D[1] to D[n]corresponding to the first switching elements 21, the LD internalresistance r_(d), and the threshold voltage V_(f0).

$\begin{matrix}{{Io} = \frac{{\sum_{k = 1}^{n}{\frac{D\lbrack k\rbrack}{r_{L}\lbrack k\rbrack} \cdot V_{in}}} - {\sum_{k = 1}^{n}{\frac{1}{r_{L}\lbrack k\rbrack} \cdot V_{f\; 0}}}}{1 + {\sum_{k = 1}^{n}{\frac{1}{r_{L}\lbrack k\rbrack} \cdot r_{d}}}}} & (5)\end{matrix}$

An appropriate memory previously stores the LD internal resistance r_(d)and the threshold voltage V_(f0), which are characteristics of the LD 2,and the control-unit internal resistances r_(L)[1] to r_(L)[n], whichare characteristics of the current control units 12-1 to 12-n, so thatthe duty ratios D[1] to D[n], 1-D[1] to 1-D[n], or the like, foroutputting the desired output current I_(o) can be calculated fromEquation (5). Thus, without using current sensors, or the like, theoutput current I_(o) can be controlled.

FIG. 8 is a diagram that illustrates the relation among the targetcurrent value Ictrl with regard to the light emitting device 1 accordingto the first embodiment, the duty ratio D, the inductor current i, andthe output current I_(o). According to this example, the target currentvalue Ictrl is changed during driving. D1 in FIG. 8 denotes the dutyratio that corresponds to the target current value Ictrl before it ischanged, and D2 denotes the duty ratio that corresponds to the targetcurrent value Ictrl after it is changed. The duty ratios D1, D2 can becalculated by using Equation (5). The duty ratios D1, D2 are applied tocontrol on the switching elements 21, 22 so that the current value ofeach of the inductor currents i is changed and the output current I_(o)can be the target current value Ictrl.

Furthermore, if the control-unit internal resistances r_(L)[1] tor_(L)[n] are identical to the duty ratios D[1] to D[n] of the currentcontrol units 12-1 to 12-n, respectively, Equation (5) can be simplifiedto the following Equation (6).

$\begin{matrix}{{Io} = \frac{{D \cdot V_{in}} - V_{f\; 0}}{\frac{r_{L}}{n} + r_{d}}} & (6)\end{matrix}$

With reference to FIGS. 9 to 11, an explanation is given below of a casewhere the current control units 12-1 to 12-n are individually stopped.

FIG. 9 is a diagram that illustrates a state where the first switchingelement 21 in the n-th current control unit 12-n according to the firstembodiment is off and the second switching element 22 is on. If currentsupply from the n-th current control unit 12-n to the LD 2 is stopped,the timing signal PWMH[n] to the first switching element 21 is set to Lso that the first switching element 21 is turned off and the connectionbetween the direct-current power source 11 and the LD 2 is blocked. Atthis time, if the second switching element 22 is on (if the timingsignal PWML[n] is H in a certain cycle), the LD 2 and the ground 25 areconnected through the inductor 23 and the second switching element 22.Therefore, in some cases, part of the output current I_(o), which is asynthesis of the inductor currents i[1], i[2] output from the first andsecond current control units 12-1, 12-2 that are being driven, leak tothe ground 25 through the inductor 23 and the second switching element22 in the n-th current control unit 12-n, the inductor current i[n] ofthe n-th current control unit 12-n becomes negative, and current supplyto the LD 2 becomes insufficient. Therefore, it is preferable that, tostop the n-th current control unit 12-n from being driven, not only thefirst switching element 21 but also the second switching element 22 isturned off.

FIG. 10 is a diagram that illustrates a state where the first switchingelement 21 and the second switching element 22 in the n-th currentcontrol unit 12-n according to the first embodiment are off. In thisway, to individually stop the n-th current control unit 12-n from beingdriven, both the first switching element 21 and the second switchingelement 22 are turned off so that the connection between the LD 2 andthe ground 25 is blocked and leak of the output current I_(o) to theground 25 can be prevented.

FIG. 11 is a timing chart that illustrates the relation among the targetcurrent value Ictrl, the duty ratio D, the timing signals PWMH, PWML,the inductor current i, and the output current I_(o) in the stateillustrated in FIG. 10. As illustrated in FIG. 11, to stop the n-thcurrent control unit 12-n from being driven, the timing signal PWMH[n]to the first switching element 21 is set to L, and also the timingsignal PWML[n] to the second switching element 22 is set to L. Duringtypical operation, as the timing signal PWML[n] to the second switchingelement 22 is an inversion signal of the timing signal PWMH[n] to thefirst switching element 21, the timing signal PWML[n] is set to H whenthe timing signal PWMH[n] is set to L. Therefore, to individually stopthe current control units 12-1 to 12-n, the calculation unit 13 setsboth the timing signals PWMH, PWML, which are input to the targetcurrent control unit, to L so that the first switching element 21 andthe second switching element 22 are simultaneously turned off. Thus,only a specific current control unit can be stopped without causing leakof the output current I_(o).

FIG. 12 is a diagram that illustrates a configuration of the lightemitting device 1 according to a modification of the first embodiment.Each of current control units 52-1 to 52-n in a drive circuit 51according to this modification is configured by using a diode 53 insteadof the above-described second switching element 22. The diode 53 is asemiconductor device that limits a current flowing direction to acertain direction to prevent the inductor current i from leaking intothe ground 25. A calculation unit 55 according to this comparativeexample generates only the timing signal PWMH for controlling the firstswitching element 21 and does not generate the above-described timingsignal PWML. With this configuration, only a specific current controlunit among the current control units 52-1 to 52-n can be stopped withoutcausing leak of the output current I_(o). Furthermore, with the drivecircuit 51 according to this modification, as there is no need togenerate the timing signal PWML to the second switching element 22,simplification of a circuit configuration, reduction in calculationloads, and the like, can be achieved.

FIG. 13 is a graph that illustrates the relation among the duty ratio D,the output current I_(o), and the number of the current control units12-1 to 12-n, 52-1 to 52-n to be driven according to the firstembodiment. The line segment corresponding to n=1 represents therelation between the duty ratio D and the output current I_(o) when oneof the current control units 12-1 to 12-n, 52-1 to 52-n is driven. Theline segment corresponding to n=2 represents the relation between theduty ratio D and the output current I_(o) when two of the currentcontrol units 12-1 to 12-n, 52-1 to 52-n are driven. The line segmentcorresponding to n=3 represents the relation between the duty ratio Dand the output current I_(o) when three of the current control units12-1 to 12-n, 52-1 to 52-n are driven. The line segment corresponding ton=4 represents the relation between the duty ratio D and the outputcurrent I_(o) when four of the current control units 12-1 to 12-n, 52-1to 52-n are driven.

D2 indicates the duty ratio needed for the output current I_(o) to reachthe current value Itarget when n=2. D3 indicates the duty ratio neededfor the output current I_(o) to reach the current value Itarget whenn=3. D4 indicates the duty ratio needed for the output current I_(o) toreach the current value Itarget when n=4. D_(max) indicates the maximumduty ratio with respect to each number to be driven. Absence of D1 inthe graph indicates that when n=1, the output current I_(o) does notreach the current value Itarget even if driving is conducted at themaximum duty ratio.

As the value of n is lager, the value of the output current I_(o)corresponding to the maximum duty ratio D_(max) is larger; therefore, itis understood that the larger output current I_(o) can be output as thenumber of the current control units 12-1 to 12-n, 52-1 to 52-n to bedriven is larger. Furthermore, because of D4<D3<D2, it is understoodthat the duty ratio D needed to obtain the current value Itarget issmaller as the number of the current control units 12-1 to 12-n, 52-1 to52-n to be driven is larger.

With reference to FIGS. 14 to 16, an explanation is given below ofoperation to maintain the output current I_(o) constant by changing theduty ratio while some of the current control units 12-1 to 12-n, 52-1 to52-n are dynamically stopped.

FIG. 14 is a timing chart that illustrates control if the number of thefour current control units 12-1 to 12-4 to be driven is sequentiallydecreased according to the first embodiment. The example illustrated inFIG. 14 illustrates a case where the target current value Ictrl isItarget and the number of the current control units 12-1 to 12-4 to bedriven is decreased from 4 to 3 and then from 3 to 2. The duty ratio isD4 when the number to be driven is 4, the duty ratio is D3 when thenumber to be driven is 3, and the duty ratio is D2 when the number to bedriven is 2. If the relation illustrated in FIG. 13 is applied,D4<D3<D2. That is, if the number to be driven is relatively large, theduty ratio used is relatively small, and if the number to be driven isrelatively small, the duty ratio used is relatively large. Thus, whenthe number of the current control units 12-1 to 12-n to be driven isdynamically changed, the current control units 12-1 to 12-n in themiddle of driving can be controlled at the duty ratio that correspondsto the number to be driven. By conducting this control, the outputcurrent I_(o) can be kept at a constant value (Itarget).

FIG. 15 is a timing chart that illustrates control if the four currentcontrol units 12-1 to 12-4 according to the first embodiment aresequentially stopped one by one. In the example illustrated in FIG. 15,the four current control units 12-1 to 12-4 are sequentially stopped,starting from the fourth current control unit 12-4, the third currentcontrol unit 12-3, the second current control unit 12-2, and then thefirst current control unit 12-1. That is, according to this example, asthe three current control units are driven on a constant basis, thedriven current control units can be driven at the duty ratio D3. Byconducting this control, the output current I_(o) can be kept at aconstant value (Itarget).

FIG. 16 is a timing chart that illustrates control if the target currentvalue Ictrl is periodically changed according to the first embodiment.In the example illustrated in FIG. 16, the target current value Ictrl isperiodically changed between 0 and Itarget, three of the four currentcontrol units 12-1 to 12-4 are driven at the duty ratio D3 when thefirst Itarget is output, and all of the four current control units 12-1to 12-4 are driven at the duty ratio D4 when the second Itarget isoutput. In this way, even if the target current value Ictrl isperiodically changed, control is performed such that the number of thecurrent control units 12-1 to 12-4 to be driven corresponds to a dutyratio so that the output current I_(o) can be kept at a desired value.

In the case described above, the number of the current control units12-1 to 12-n is 4; however, the same control may be performed if thenumber of the current control units 12-1 to 12-n is other than 4.Furthermore, in the example described, the direct-current power source11 is used as a power source; however, an alternating-current powersource may be used. Moreover, in the example described, a laser diode(LD) is used as a light emitting element; however, the type of lightemitting element is not particularly limited, and for example a lightemitting diode (LED) may be used.

As described above, according to the present embodiment, multiplecurrent control units are connected in parallel, including inductors andbeing driven by a switching control method, so that high output currentscan be achieved without increasing the size of the inductor. Thus, theoutput current of a large current value (e.g., a few hundred A) can beoutput without causing magnetic saturation in the inductor. Furthermore,the drive states of the current control units are individuallycontrolled in accordance with the target current value of the outputcurrent so that high outputs can be achieved while high electric-powerconversion efficiency is retained.

An explanation is given below of other embodiments with reference to thedrawings, and the parts for producing the function effect that is thesame as or similar to that in the first embodiment are attached with thesame reference numerals and their explanations are omitted.

Second Embodiment

FIG. 17 is a diagram that illustrates a configuration of the lightemitting device 1 according to a second embodiment. A drive circuit 71according to the present embodiment has a failure detection function anda failure handling function. The failure detection function is afunction to identify the faulty current control units 12-1 to 12-n inaccordance with fluctuation in the output current I_(o) when the currentcontrol units 12-1 to 12-n to be driven or stopped are sequentiallychanged. The failure handling function is a function to output therequested output current I_(o) by controlling the normal current controlunits 12-1 to 12-n even when any of the current control units 12-1 to12-n is faulty.

The drive circuit 71 according to the present embodiment includes asensor 72 (current detection means) that detects the output currentI_(o). A calculation unit 75 according to the present embodimentidentifies the faulty current control units 12-1 to 12-n in accordancewith a detection current value Isens detected by the sensor 72, stopsthe faulty current control units 12-1 to 12-n, and controls the firstswitching element 21 and the second switching element 22 in the normalcurrent control units (the current control units other than the faultycurrent control unit) 12-1 to 12-n so that the output current I_(o)becomes the target current value Ictrl.

FIG. 18 is a timing chart that illustrates operation when the drivecircuit 71 according to the second embodiment performs a failuredetection function. In the example illustrated in FIG. 18, the fourcurrent control units 12-1 to 12-4 are sequentially stopped one by one.According to this example, a schedule is determined such that the fourthcurrent control unit 12-4, the third current control unit 12-3, thesecond current control unit 12-2, and the first current control unit12-1 are stopped in this order. According to this example, after thefourth current control unit 12-4 is stopped, the third current controlunit 12-3 is stopped, and when the fourth current control unit 12-4starts to be driven, the inductor current i[4] of the fourth currentcontrol unit 12-4 is lower than an ideal value 101. Therefore, the valueof the output current I_(o) when the fourth current control unit 12-4starts to be driven is lower than an ideal value 102. Such a fluctuationof the output current I_(o) is detected by the sensor 72 and isrecognized by the calculation unit 75. That is, the calculation unit 75can detect an error between the detection current value Isens detectedby the sensor 72 and the ideal value 102 and determine that a failureoccurs in the current control units 12-1 to 12-4 that start (restart) tobe driven in timing in which the error is detected. According to thisexample, it is determined that the fourth current control unit 12-4 hasa failure. Furthermore, in the example described, a circumstance wherethe output current I_(o) is lower than the ideal value 102 is a failure;however, the fluctuation in the output current I_(o) indicating afailure is not limited thereto, and for example there may be acircumstance where the output current I_(o) is higher than the idealvalue 102.

FIG. 19 is a timing chart that illustrates operation when the drivecircuit 71 according to the second embodiment performs the failurehandling function. In the example illustrated in FIG. 19, if a failureof the fourth current control unit 12-4 is detected as described above,duty-ratio control is conducted on only the normal current control units12-1 to 12-3 as targets. According to the original schedule, when thethird current control unit 12-3 is stopped, the first, second, andfourth current control units 12-1, 12-2, and 12-4 are driven at the dutyratio D3; however, with the failure handling function according to thisexample, the fourth current control unit 12-4 is stopped, and the firstand second current control units 12-1, 12-2 are driven at the duty ratioD2. By this control, the requested output current I_(o) can be outputwithout being affected by the faulty fourth current control unit 12-4.

According to the above-described embodiment, without providing a failuredetection means in each of the current control units 12-1 to 12-n, thefaulty current control units 12-1 to 12-n can be identified and properlyhandled. Furthermore, as failures can be handled by conducting onlyduty-ratio control on the normal current control units 12-1 to 12-n, theoutput current I_(o) can be promptly corrected after a failure occurs.

Third Embodiment

FIG. 20 is a diagram that illustrates a configuration of the lightemitting device 1 according to a third embodiment. A drive circuit 81according to the present embodiment includes block mechanisms 82A, 82Bbefore and after each of the current control units 12-1 to 12-n. Theblock mechanisms 82A, 82B are circuits that block electric connectionbetween the direct-current power source 11 and the LD 2 and that may beconfigured by using, for example, a relay or MOSFET. The blockmechanisms 82A, 82B are used to block the faulty current control units12-1 to 12-n from an electric pathway.

A calculation unit 85 according to the present embodiment has afaulty-part block function in addition to the failure detection functionand the failure handling function described in the second embodiment.The faulty-part block function is a function to control the blockmechanisms 82A, 82B so as to block the current control units 12-1 to12-n in which a failure has been detected from an electric pathway. Thecalculation unit 85 according to this example outputs a block signal BRto the block mechanisms 82A, 82B connected before and after the currentcontrol units 12-1 to 12-n in which a failure has been detected by thefailure detection function. After receiving the block signal BR, theblock mechanisms 82A, 82B perform operation to block an electricconnection. After blocking the faulty current control units 12-1 to12-n, the calculation unit 85 conducts duty-ratio control on theremaining current control units (normal current control units) 12-1 to12-n. Thus, the drive circuit 81 (the light emitting device 1) can becontinuously driven.

As described above, the faulty current control units 12-1 to 12-n areblocked from an electric pathway so that the faulty current controlunits 12-1 to 12-n can be safely removed and replaced. Furthermore, asdriving is continuously enabled by using the normal current controlunits 12-1 to 12-n after blocking, the faulty current control units 12-1to 12-n can be handled without stopping the light emitting device 1 frombeing driven.

The embodiments of the present invention have been described above;however, the above embodiments are presented as examples, and there isno intension to limit the scope of the invention. The novel embodimentsmay be implemented as other various embodiments, and various omission,replacement, modification, and combination are possible withoutdeparting from the spirit of the invention. The embodiments and theirmodifications are included in the scope and spirit of the invention, andthey are included in the scope of the invention described in claims andtheir equivalents.

REFERENCE SIGNS LIST

-   -   1 Light emitting device    -   2 LD (light emitting element)    -   3, 51, 71, 81 Drive circuit    -   11 Direct-current power source (power source)    -   12-1 to 12-n, 52-1 to 52-n Current control unit    -   13, 55, 75, 85 Calculation unit    -   14 Capacitor    -   21 First switching element    -   22 Second switching element    -   23 Inductor    -   24 Contact point    -   25 Ground    -   53 Diode    -   72 Sensor (current detection unit)    -   82A, 82B Block mechanism    -   101, 102 Ideal value    -   BR Block signal    -   D Duty ratio (corresponding to first switching element)    -   i Inductor current    -   Ictrl Target current value    -   I_(o) Output current    -   Isens Detection current value    -   L Inductance    -   PWMH, PWML Timing signal (pulse modulation signal)    -   r_(d) LD internal resistance    -   r_(L) Control-unit internal resistance    -   V_(f) Forward voltage    -   V_(in) Input voltage

CITATION LIST Patent Literature

-   PTL 1: Japanese Patent No. 6009132

1. A drive circuit configured to generate an output current for drivinga light emitting element, comprising: a power source; a current controlunit configured to control an amount of current supplied to the lightemitting element in accordance with a pulse modulation signal; and acalculation unit configured to change a duty ratio of the pulsemodulation signal, wherein the current control unit includes a firstswitching element configured to be switched on/off in accordance withthe pulse modulation signal; a second switching element configured to beswitched on/off in accordance with an inversion signal of the pulsemodulation signal input to the first switching element; and an inductor,the first switching element and the inductor are serially connectedbetween the power source and the light emitting element, the secondswitching element is connected between ground and a contact point of thefirst switching element and the inductor, and the two or more currentcontrol units are connected in parallel.
 2. The drive circuit accordingto claim 1, wherein the calculation unit adjusts the duty ratio inaccordance with the output current, a voltage of the power source, anumber of the current control units, an internal resistance of thecurrent control unit, an internal resistance of the light emittingelement, and a threshold voltage due to a potential barrier of the lightemitting element.
 3. The drive circuit according to claim 1, wherein thecalculation unit calculates the duty ratio in accordance with Equation(1) described below, where the output current is I_(o), a voltage of thepower source is V_(in), a number of the current control units is n, aninternal resistance of the current control unit is r_(L)[k], the dutyratio that corresponds to the first switching element is D[k], aninternal resistance of the light emitting element is r_(d), and athreshold voltage due to a potential barrier of the light emittingelement is V_(fO). $\begin{matrix}{{Io} = \frac{{\sum_{k = 1}^{n}{\frac{D\lbrack k\rbrack}{r_{L}\lbrack k\rbrack} \cdot V_{in}}} - {\sum_{k = 1}^{n}{\frac{1}{r_{L}\lbrack k\rbrack} \cdot V_{f\; 0}}}}{1 + {\sum_{k = 1}^{n}{\frac{1}{r_{L}\lbrack k\rbrack} \cdot r_{d}}}}} & (1)\end{matrix}$
 4. The drive circuit according to claim 1, wherein theoutput current satisfies Equation (2) described below, where the outputcurrent is I_(o), a voltage of the power source is V_(in), a number ofthe current control units is n, an internal resistance of the currentcontrol unit is r_(L)[k], the duty ratio that corresponds to the firstswitching element is D[k], an internal resistance of the light emittingelement is r_(d), and a threshold voltage due to a potential barrier ofthe light emitting element is V_(fO). $\begin{matrix}{{Io} = \frac{{\sum_{k = 1}^{n}{\frac{D\lbrack k\rbrack}{r_{L}\lbrack k\rbrack} \cdot V_{in}}} - {\sum_{k = 1}^{n}{\frac{1}{r_{L}\lbrack k\rbrack} \cdot V_{f\; 0}}}}{1 + {\sum_{k = 1}^{n}{\frac{1}{r_{L}\lbrack k\rbrack} \cdot r_{d}}}}} & (2)\end{matrix}$
 5. The drive circuit according to any one of claims 1 to4, wherein the calculation unit individually stops at least one of thecurrent control units.
 6. The drive circuit according to claim 5,wherein when the current control unit is to be stopped, the calculationunit switches off the first switching element and the second switchingelement in the current control unit to be stopped.
 7. The drive circuitaccording to claim 5 or 6, further comprising a current detection unitconfigured to detect the output current, wherein the calculation unitidentifies the current control unit that is faulty in accordance with afluctuation in the output current detected when the current control unitto be stopped is switched.
 8. The drive circuit according to claim 7,wherein when the faulty current control unit is identified, thecalculation unit stops the faulty current control unit and controls theduty ratio for the current control unit that is normal other than thefaulty current control unit.
 9. The drive circuit according to claim 7or 8, further comprising a block unit configured to block the faultycurrent control unit from an electric pathway.
 10. A drive circuitconfigured to generate an output current for driving a light emittingelement, comprising: a power source; a current control unit configuredto control an amount of current supplied to the light emitting elementin accordance with a pulse modulation signal; and a calculation unitconfigured to change a duty ratio of the pulse modulation signal,wherein the current control unit includes a switching element configuredto be switched on/off in accordance with the pulse modulation signal; aninductor; and a diode configured to limit flow of a current from theinductor to ground, wherein the switching element and the inductor areserially connected between the power source and the light emittingelement, the diode is connected between the ground and a contact pointof the switching element and the inductor, and the two or more currentcontrol units are connected in parallel.
 11. A light emitting devicecomprising a light emitting element configured to be driven with anoutput current generated by the drive circuit according to any one ofclaims 1 to 10.